1. Field of the Invention
The present invention relates to a computed-tomography (CT) system configured to reconstruct the tomographic image of a subject based on projection-image data obtained by radiating radiation including an X-ray, etc. to the subject.
2. Description of the Related Art
CT apparatuses have been significantly advancing ever since they were invented. In recent years, the helical-scan method has been put to practical use, for example, so as to radiate an X-ray to a subject in a helical fashion by making an X-ray tube circle the periphery of the subject continually while moving the subject in the body-axis direction. Further, apparatuses that can perform tomographic imaging for obtaining slice images through a single rotation by using a two-dimension detector have been put to practical use.
CT apparatuses referred to as so-called third-generation CT apparatuses usually capture a tomographic image through the following procedures. First, an X-ray tube and a multi-channel X-ray detector are provided so that they are opposed to each other and a subject is provided therebetween. An X-ray beam is radiated from the X-ray tube to the subject while making the X-ray tube and the X-ray detector rotate 360 degrees around the subject, and the X-ray that had passed through the subject is detected by the X-ray detector. At that time, the intensity of the X-ray radiated from the X-ray tube is constant (namely, the tube voltage and the tube current of the X-ray tube is constant). Here, the X-ray emitted from the focal point of the X-ray tube is collimated with a fan-shaped X-ray beam. Further, the spread width of the X-ray beam is determined based on the slice thickness, etc.
The above-described operations performed to radiate the X-ray from the X-ray tube to the subject by making the X-ray tube and the X-ray detector rotate around the subject and detect the X-ray transmitted through the subject by using the X-ray detector are referred to as “scanning”. Further, a rotation angle at which the X-ray transmitted through the subject is detected (sampled) by using the X-ray detector during the above-described rotation is referred to as a “view angle”. The tomographic image of the subject can be acquired by reconstructing the projection-image-data items corresponding to a plurality of views obtained through the above-described scanning.
Since the above-described X-ray CT apparatuses of late can capture many fine slice images at the same time, the X-ray-CT apparatuses have to reconstruct many slice images with high speed by using the projection-image-data items corresponding to captured views.
At present, Japanese Patent Laid-Open No. 2005-334110 discloses an example method of distributing the above-described reconstruction processing.
Thus, there have been requests for reconstructing many slice images due to the advance of the X-ray CT apparatuses. Therefore, pre-processing, convolution-calculation processing, and post-processing that have been considered as insignificant are put under heavy processing loads. Therefore, there has been the task of performing not only back-projection-calculation processing, but also the pre-processing, the convolution-calculation processing, and the post-processing with high speed.